That still makes 5...
Relief is an idea, but I was thinking more about features, such as rivers, mountains and deserts that form natural borders, like a lot of European countries have. I know that some states for example follow the Mississipi river, but from a European's perspective honestly it's boring. I think designing a map without the states in mind would be really intersting to a non-American especially. We could easily make it a "What if America was colonised 1000 years ago" or "Colonise America" (could be really intersting to recreate the Gold Rush, for example) or even "Create your own state" like we have "create your own nation" on Europe maps.
Basically, imagine a european map, but using the geography of America :)
Something along these lines: